In the art of injection molding, a typical injection mold will contain one or more mold cores and cavities that correspond to the shape of the molded article being produced. A melt stream of moldable material is injected from an injection molding machine into the mold cavities through a hot runner system, where it is allowed to solidify for a period of time before the mold is opened and the newly molded parts are ejected. One of the most significant factors in affecting the overall cycle time required to produce one or a plurality of molded articles is the time required to solidify or cool the newly molded articles within the mold cavity before the parts are ejected.
In injection molding applications such as the molding of polyethylene terephthalate (PET) preforms, the ability to rapidly cool the molded articles in the mold is of utmost importance since the newly molded preforms are in many instances removed from the mold by a robotic take-out plate as soon as they have solidified to a point where they can be handled without being damaged, and are then further cooled for a number of additional molding cycles by a post mold cooling apparatus. Generally, post mold cooling systems are configured to cool only the interior and exterior of the body of the preform and as such the molding cycle time is generally limited by the ability to sufficiently cool the thread/neck portion of the preform while it is still in the mold.
In PET molding, an assembly of components, known in the art as a mold stack, defines the cavity in which the preform is molded. The threaded neck portion of the preform is formed within the mold cavity by a complementary pair of mating mold halves known as neck rings or split thread inserts. Along with providing the molding surface which defines the threaded neck portion of the preform, the pair of split thread inserts are also used to strip the preform from the mold core during ejection of the preform from the mold, and are further used to align the core and cavity with respect to one another when the two halves of the mold are clamped together.
Due to their location within the mold stack, along with the various functions required of the split thread inserts, mold builders face great difficulty in providing adequate cooling within the split thread inserts adjacent the molding surface. Cooling channels that are usually manufactured at some distance from the molding surface may not have a uniform distribution around the molding surface. Inadequate and uneven cooling may result in a molded preform with quality problems such as an undesirable oval shape, dimensions out of specification, and deformations. In order to avoid these problems, cycle time is usually increased to allow longer cooling and more plastic solidification. However, longer cycle times impacts the manufacturing process by causing lower productivity and increases the cost of plastic parts produced.
There are many ways of designing and manufacturing split thread inserts with cooling channels. Some manufacturers attempt to provide uniform cooling circuitry inside of each of the split thread inserts by forming the split thread insert from more than one piece with the mating surfaces between the pieces sometimes having a very complex cut, which can make joining the pieces together to form the split thread insert more difficult. Such multi-piece split thread inserts may not be cost effective or practical for forming smaller molded parts due to the added complexity of the design/shape. As such a need still exists in the art for split thread inserts having internal cooling circuitry that are relatively simple to manufacture and which can provide rapid cooling of the threaded neck portion of any size preform.